Re: [PATCH v2 1/2] KVM: arm64: Validate the host vCPU's VM before reading it under pKVM

[Joey Gouly <joey.gouly@arm.com>]

Hi,

On Tue, Sep 15, 2026 at 08:04:17AM +0100, Fuad Tabba wrote:
> On an MTE-capable host under pKVM, enter_exception64() reads the VM's
> MTE flag through vcpu->kvm, which for a host vCPU is a host-writable
> pointer nothing validates. The host can point it at any address in the
> hyp linear map and read back bit 1 of that word through PSR_TCO in the
> vCPU's CPSR, or panic the hypervisor with an unmapped one.
> 
> Get the VM through a get/put pair around the read: a loaded vCPU's is
> the hyp VM, an unloaded host vCPU's is read once and pinned, and a
> pointer the host never shared leaves TCO clear.
> 
> Fixes: ea7fc1bb1cd1b ("KVM: arm64: Introduce MTE VM feature")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/all/20260914070536.877D91F000FF@smtp.kernel.org/
> Cc: stable@vger.kernel.org
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
>  arch/arm64/kvm/hyp/exception.c             |  5 ++-
>  arch/arm64/kvm/hyp/include/hyp/adjust_pc.h | 18 ++++++++++
>  arch/arm64/kvm/hyp/nvhe/pkvm.c             | 39 ++++++++++++++++++++++
>  3 files changed, 61 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
> index 754e2dc1df54a..6e60d890afa4a 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
> @@ -70,6 +70,7 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
>  			      enum exception_type type)
>  {
>  	unsigned long sctlr, vbar, old, new, mode;
> +	struct kvm *kvm;
>  	u64 exc_offset;
>  
>  	mode = *vcpu_cpsr(vcpu) & (PSR_MODE_MASK | PSR_MODE32_BIT);
> @@ -109,8 +110,10 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
>  	new |= (old & PSR_C_BIT);
>  	new |= (old & PSR_V_BIT);
>  
> -	if (kvm_has_mte(kern_hyp_va(vcpu->kvm)))
> +	kvm = vcpu_get_kvm(vcpu);
> +	if (kvm && kvm_has_mte(kvm))

It's fine to use the "host->kvm" here because even if sets this bit on
the host vCPU, the per-entry handlers in the other series won't copy
across the TCO bit.
Right?

>  		new |= PSR_TCO_BIT;
> +	vcpu_put_kvm(vcpu, kvm);
>  
>  	new |= (old & PSR_DIT_BIT);
>  

[snip]

Agree with the nits that Vincent suggested!

Let's see if this sticks this time:
Reviewed-by: Joey Gouly <joey.gouly@arm.com>

Thanks,
Joey